Indirectly heated cathode for discharge tubes



Oct. 1-3, 1936. K. VAN GESSEL ET AL INDIRECTLY HEATED CATHODE FORDISCHARGE TUBES Filed May 2, 1934 INVENTORS J0 AE/P ATTORNEY A. M. Z4GESISEL G/LLEJ 1901.57

40UADAG PASTE M 6 N m m Patented Oct. 13, 1936 PATENT OFFICE INDIRECTLYHEATED CATHODE FOR DISCHARGE TUBES Karel Marinus Van Gessel, GillesHolst,

and

Johan Lodewyk Hendrik Jonker, Eindhoven, Netherlands, assignors to RadioCorporation of America, a corporation of Delaware Application May 2,1934, Serial No. 723,510 In the Netherlands July 13, 1933 3 Claims.

This invention relates to an indirectly heated cathode for electricdischarge tubes and more particularly to cathodes consisting of aheating element surrounded by a cathode sleeve or body, insulatingmaterial being provided, if desired, between said members.

A well known phenomenon occurring when using discharge tubes comprisingcathodes of this type is that it takes often very long before thecathode has been heated from the heating member to the temperaturerequired for emission. This phenomenon manifests itself in that often aconsiderable time elapses before sound is reproduced by a radio setafter it has been switched We have now succeeded in constructing acathode by means of which this drawback is entirely avoided or at leastreduced to a great extent. According to the invention the cathodeheating member and/or the side of the cathode body facing said heatingmember is coated with a substance having a large heat radiatingcapacity. It is also possible to make one or both of said membersentirely or partly of such a material. We have found it advantageous tocoat these surfaces with a substance acting perfectly as a black bodyand for this purpose vanadium trioxide is preferably used. However, theeffect aimed at by the invention can also be achieved by treating saidsurfaces with nickel oxide or carbon. By means of this construction itis ensured that there are one or more surfaces having a large heatradiating capacity between the heating member and the cathode body. Infact, when providing insulating material between the heating member andthe cathode it is advisable to coat this material on one or both sideswith a material having a large heat radiating capacity. Due to theseconstructions the time required for heating the cathode to the emissiontemperature is greatly reduced.

The constructions described above are more particularly adapted for usewith cathodes the cathode sleeves of which consist entirely or partly ofa substance, for instance, copper or silver, whose heat radiatingcapacity is smaller than that of nickel. When using such materialswithout the aid of our invention the time required for reaching theemission temperature is often very considerable. Indeed, in this case acomparatively small quantity of energy is suiiicient for heating thecathode to a definite temperature. Since, however, the cathode must beheated to the same temperature as a cathode body consisting, forinstance, of nickel, the time required for reaching this temperature isusually fairly considerable. Therefore, it is desired especially withthese cathodes to provide means by which the time required for heatingis reduced.

The invention will be more clearly understood by reference to theaccompanying drawing representing, by way of example, one embodimentthereof.

A rod l consisting, for instance, of magnesium oxide is immersed inAquadag paste so that it is covered with a thin layer 2 of said paste.This rod is introduced into a cathode sleeve 3 which may be of copper,nickel or other suitable material and turned about a few times so as tointimately contact with the inner wall thereof. After removing the rodand after drying the cathode sleeve at room temperature, the inner Wallof the sleeve is coated with a uniform black layer 4. If desired theAquadag paste may be permitted to flow through the long cathode tubefrom which the cathode sleeves are out.

After this treatment an insulating rod or tube of, for instance,magnesium oxide or aluminum oxide, is inserted in the cathode sleeve ortube and the heater element 5 of filamentary form, which may itself becoated with insulating material, if desired, is then threaded throughsaid insulating rod or tube, whereupon the cathode thus formed togetherwith the other electrodes are mounted within the evacuated envelope, theother operations required for the manufacture of the tube being effectedin the usual manner.

Another method within the spirit of the present invention is to smearthe insulating tube which is to be provided between the heating memberand the cathode sleeve and tube with graphite, or to blacken it bycoating with a layer of vanadium trioxide. The insulating tube thuscoated and having the heater threaded therethrough is then insertedwithin the cathode tube, further mounting thereof and other electrodesbeing effected in a normal manner.

It appears from what has been described above that various forms ofcathode construction according to the present invention are possible andit is therefore desired not to be limited except by the appended claims.

What we claim is:

1. An indirectly heated cathode for electron discharge tubes comprisinga tubular electron emitting member of metal having a lower heatradiating capacity than nickel, and means for increasing the heatradiating capacity of said tubular member comprising a coating ofvanadium trioxide on the inner surface of said memher.

2. An indirectly heated cathode for electron discharge tubes comprisinga tubular electron emitting member of copper, and means for increasingthe heat radiating capacity of said member, comprising a coating ofvanadium trioxide on the inner surface of said copper member.

3. An indirectly heated cathode for electron" discharge tubes comprisinga tubular electron emitting member of silver, and means for increasingthe heat radiating capacity of said member, comprising a coating ofvanadium trioxide on the inner surface of said silver member.

KAREL MARINUS VAN GESSEL. GILLES HOLST. JOI-IAN LODEWYK HENDRIK JONKER.

